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Page 56
A general acquaintance with the stars will be first attempted;
a more particular knowledge afterward. Fig. 67 (page 201) is a
map of the circumpolar region, which is in full view every clear
night. It revolves daily round Polaris, its central point. Toward
this star, the two end stars of the Great Dipper ever point, and
are in consequence called "the Pointers." The map may be held toward
the northern sky in such a position as the stars may happen to be.
The Great Bear, or Dipper, will be seen at nine o'clock in the
evening above the pole in April and May; west of the pole, the
Pointers downward, in July and August; close to the north horizon
in October and November; and east of the pole the Pointers highest,
in January and February. The names of such constantly visible stars
should be familiar. In order, from the end of the tail of the Great
Bear, we have Benetnasch ae, Mizar z, Little Alcor close to it,
[Page 198] Alioth, e Megrez, d at the junction, has been growing
dimmer for a century, Phad, g Dubhe and Merak. It is best to get
some facility at estimating distances in degrees. Dubhe and Merak,
"the Pointers," are five degrees apart. Eighteen degrees forward of
Dubhe is the Bear's nose; and three pairs of stars, fifteen degrees
apart, show the position of the Bear's three feet. Follow "the
Pointers" twenty-nine degrees from Dubhe, and we come to the
pole-star. This star is double, made of two suns, both appearing as
one to the naked eye. It is a test of an excellent three-inch
telescope to resolve it into two. Three stars beside it make the
curved-up handle of the Little Dipper of Ursa Minor. Between the two
Bears, thirteen degrees from Megrez, and eleven degrees from Mizar,
are two stars in the tail of the Dragon, which curves about to
appropriate all the stars not otherwise assigned. Follow a curve of
fifteen stars, doubling back to a quadrangle from five to three
degrees on a side, and thirty-five degrees from the pole, for his
head. His tongue runs out to a star four degrees in front. We shall
find, hereafter, that the foot of Hercules stands on this head. This
is the Dragon slain by Cadmus, and whose teeth produced such a crop
of sanguinary men.
The star Thuban was once the pole-star. In the year B.C. 2300 it
was ten times nearer the pole than Polaris is now. In the year
A.D. 2100 the pole will be within 30' of Polaris; in A.D. 7500,
it will be at a of Cepheus; in A.D. 13,500, within 7� of Vega; in
A.D. 15,700, at the star in the tongue of Draco; in A.D. 23,000,
at Thuban; in A.D. 28,000, back to Polaris. This indicates no change
in the position of the dome [Page 199] of stars, but a change in the
direction of the axis of the earth pointing to these various places
as the cycles pass. As the earth goes round its orbit, the axis,
maintaining nearly the same direction, really points to every part
of a circle near the north star as large as the earth's orbit, that
is, 185,000,000 miles in diameter. But, as already shown, that
circle is too small to be discernible at our distance. The wide
circle of the pole through the ages is really made up of the
interlaced curves of the annual curves continued through 25,870
years. The stem of the spinning top wavers, describes a circle, and
finally falls; the axis of the spinning earth wavers, describes a
circle of nearly 28,000 years, and never falls.
The star g Draconis, also called Etanin, is famous in modern astronomy,
because observations on this star led to the discovery of the
_aberration of light_. If we held a glass tube perpendicularly out
of the window of a car at rest, when the rain was falling straight
down, we could see the drops pass directly through. Put the car
in motion, and the drops would seem to start toward us, and the
top of the tube must be bent forward, or the drops entering would
strike on the backside of the tube carried toward them. So our
telescopes are bent forward on the moving earth, to enable the
entered light to reach the eye-piece. Hence the star does not appear
just where it is. As the earth moves faster in some parts of its
orbit than others, this aberration is sometimes greater than at
others. It is fortunate that light moves with a uniform velocity,
or this difficult, problem would be still further complicated.
The displacement of a star from this course is about 20".43.
[Page 200]
On the side of Polaris, opposite to Ursa Major, is King Cepheus,
made of a few dim stars in the form of the letter K. Near by is
his brilliant wife Cassiopeia, sitting on her throne of state.
They were the graceless parents who chained their daughter to a
rock for the sea-monster to devour; but Perseus, swift with the
winged sandals of Mercury, terrible with his avenging sword, and
invincible with the severed head of Medusa, whose horrid aspect of
snaky hair and scaly body turned to stone every beholder, rescues
the maiden from chains, and leads her away by the bands of love.
Nothing could be more poetical than the life of Perseus. When he
went to destroy the dreadful Gorgon, Medusa, Pluto lent him his
helmet, which would make him invisible at will; Minerva loaned
her buckler, impenetrable, and polished like a mirror; Mercury
gave him a dagger of diamonds, and his winged sandals, which would
carry him through the air. Coming to the loathsome thing, he would
not look upon her, lest he, too, be turned to stone; but, guided
by the reflection in the buckler, smote off her head, carried it
high over Libya, the dropping blood turning to serpents, which
have infested those deserts ever since.
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